1. Cloned human cytotoxic cells (CTL) have been retargeted by using antibody heteroaggregates containing anti-T3 cross linked to anti-target cell antibodies. We have shown that by using this technique cloned CTL lose their normal specificity and can be made to lyse allogeneic and xenogeneic tumor cells, and even chicken red blood cells. These results suggest that effector cell retargetting could be used in vivo to treat neoplasms and other pathogens which express distinctive surface antigens. 2. In order to obtain effector cells which might be used in vivo to inflict cell mediated death upon tumor and other pathogenic cells, human peripheral blood leukocytes have been treated with anti-T3 containing antibody heteroaggregates and tested for the ability to lyse tumor cells expressing an antigen recognized by the second antibody of the heteroaggregate. Human peripheral blood T-cells when treated in this way are potent and specific mediators of target cell lysis. This is true without prior activation, but brief exposure to recombinant IL-2 rapidly increases the cytolytic activity to an even higher level. The effector cells in this case are T8+.